Regulations relating to fire safety have become more stringent in recent years. For example, one aspect of relatively new Federal Regulations relating to handicapped persons is the requirement for visual fire warning systems for the hearing-impaired in various types of public spaces. Visual alarms using strobe lamps are well-known and in widespread use, primarily in industrial and vehicular settings. They will in the future be more frequently installed in public spaces in institutional and commercial buildings, not only when required but as a preferred safety practice.
Strobe lamps are elongated tubes. As such, the light they produce is not uniform in all directions. FIG. 7 of the accompanying drawings shows the light output of a bare strobe lamp (no reflector or lens) at angles in a plane perpendicular to the lamp axis relative to a line in that plane (curve 1) and at angles in a plane that includes the lamp axis relative to a line in that plane perpendicular to the lamp axis (curve 2). The light that radiates perpendicularly from the lamp relative to its axis (curve 1) is of uniform intensity. However, the light output in the plane of the lamp axis (curve 2) diminishes substantially as a function of the cosine of the angle between the radial line in the plane and all other lines. Near and at + and -90.degree., i.e. when the lamp is observed from either end, the light output is but a small fraction of its output in the radial direction.
Underwriters Laboratories (UL.RTM.) has recently adopted standards that require certain levels of light output from strobe alarm lights for fire safety warning systems in (1) a plane perpendicular to the lamp axis and (2) the plane that is perpendicular to that plane and includes the lamp axis. (Throughout this specification, the term "light" refers to a system that consists of a strobe lamp bulb, a support that may include a reflector, and a lens, and the term "lamp" refers to the light-emitting strobe light bulb element.) Curve 3 in FIG. 7 depicts the UL.RTM. requirement for plane 2 above. Meeting the requirement (not shown) for plane 1 above is not a problem, provided the lamp has the required intensity for its particular UL.RTM. rating. Meeting the requirement for plane 2 is a problem, as is apparent from FIG. 7; note that the light output (curve 2) for a bare lamp in plane 2 is below the UL.RTM. requirement at and near + and -90.degree..
One way of meeting the requirement is to equip the strobe light with two lamps, one perpendicular to the other. That way is costly and also requires more electric current, which for a given battery storage capacity reduces the operating time of the device or for a given operating time requires more battery storage capacity, another additional cost. Another possible way of meeting the UL.RTM. requirement for plane 2 is with a specially designed lens, but that means diverting light and thus requires reducing the light output in directions other than plane 2.